<p>Tryptophan synthase (<db_xref db="EC" dbkey="4.2.1.20"/>) catalyzes the last step in the biosynthesisof tryptophan [<cite idref="PUB00000116"/>, <cite idref="PUB00000769"/>]:<reaction>L-serine + 1-(indol-3-yl)glycerol 3-phosphate = L-tryptophan + glyceraldehyde 3-phosphate + H<sub>2</sub>O</reaction>It has two functional domains, each found in bacteria and plants on aseparate subunit. In <taxon tax_id="562">Escherichia coli</taxon>, the 2 subunits, A and B, are encoded by the trpA and trpB genes respectively. The alpha chain is for the aldol cleavage of indoleglycerol phosphate to indole and glyceraldehyde 3-phosphate and the beta chain <db_xref db="INTERPRO" dbkey="IPR006653"/> is for the synthesis of tryptophan from indole and serine. In fungi the two domains are fused together in a single multifunctional protein, in the order: (NH2-A-B-COOH) [<cite idref="PUB00014824"/>, <cite idref="PUB00004683"/>]. The two domains of the <taxon tax_id="5141">Neurospora crassa</taxon> polypeptide are linked by a connector of 54-amino acid residues that has less than 25% identity to the 45-residue connector of the <taxon tax_id="4932">Saccharomyces cerevisiae</taxon> (Baker's yeast) polypeptide. Two acidic residues are believed to serve as proton donors/acceptors in the enzyme'scatalytic mechanism.</p>